A number of methods and devices are already known which describe frequency conversion of continuous laser radiation by means of non-linear crystals, in particular the generation of the 2nd harmonic, with the goal of increasing the conversion efficiency.
The "classical apparatus" for frequency conversion of laser radiation, as is described in the publications by M. Brieger et al., "Enhancement of Single Frequency SHG in a Passive Ring Resonator", Opt. Commun. 38 (1981) p. 423; C. S. Adams et al. "Tunable narrow linewidth ultra-violet light generation . . . ", Opt. Commun. 90 (1992), p. 89; S. Bourzeix et al.: "Efficient frequency doubling of a continuous wave . . . ", Opt. Commun. 99 (1993), p.89, consists of a resonator in the shape of a double-Z, which is formed from four mirrors, of which at least two feature a radius of curvature, and one non-linear crystal. A first mirror is mounted to a piezo-element and is used for tuning the resonator length to resonate with the incident light wave. The portion of the incident wave reflected from a third mirror is recorded with a detector. Using standard methods (Hansch-Couillaud, Pound-Drever), a control signal can be obtained from this for active resonator stabilization. The mirror spacings, radii of curvature, coatings and the crystal are configured so that:
a) The resonator is optically stable. PA1 b) A beam waist forms between the two curved mirrors at the site of the non-linear crystal; the size of this waist is optimum for an efficient conversion. PA1 c) The astigmatism of the second beam waist between another two mirrors (third and fourth mirrors) caused by the curved mirror, is compensated by the Brewster-cut crystal. PA1 d) Three of the mirrors have the highest possible reflectivity for the fundamental wave. PA1 e) One of the mirrors for the generated harmonic has the highest possible transmissivity. PA1 f) The reflectivity of the incoupling mirror is sized so that the resonance enhancement of the fundamental wave is as great as possible, which is the case for impedance matching R=1-V(R: reflectivity, V: passive resonator losses). PA1 g) The condition for phase matching is satisfied for the non-linear crystal. PA1 a) They are mechanically much more stable and thus have a lesser susceptibility to external interference. PA1 b) They have reduced losses due to a smaller number of interfaces in the resonator. PA1 c) Expensive, precision optical components can be omitted.
With apparatus of this type, typically conversion efficiencies between 10% and 30% are attained. Since four adjustable mirror holders are needed in this configuration, the mechanical expense is relatively high. Since high-reflectivity mirrors always have a residual transmission, the passive losses from this array cannot be reduced indefinitely, so that the enhancement factor of the resonator will have an upper limit.
If the resonator tuning is performed in this array through translation of a mirror, then several disadvantages appear: The maximum permissible translation is limited by the increasing misalignment of the resonator caused by the changed beam path. An additional misalignment occurs due to the tilting occurring in standard piezoelements, which superposes the translational motion. Thus the resonator can follow a continuous variation of the incident light frequency only over a limited frequency interval (fast, continuous frequency tuning is required, e.g., for laser cooling of atoms).
Another disadvantage of the mirror translation occurs when the mirrors of the resonator have to be replaced for refitting to another wavelength range. In order to do this with a minimum expense, the mirror must be mounted using a replaceable mount to the piezoelement whose additional mass deteriorates the control dynamics for the resonator stabilization.
Usually angle-tuned crystals are used for tunable frequency conversion systems, since they will cover the largest wavelength range. When changing the wavelength of the incident radiation, the crystal must be turned in order to satisfy the condition for phase matching. During this rotation, the laser beam undergoes a parallel shift in the resonator, which necessitates a readjustment of the resonator. The beam position of the resultant harmonic is changed considerably during this readjustment. In applications that require a constant beam position, the position must be corrected again by using a downstream deflection system, such as a periscope made of two adjustable mirrors.
In other publications, for example, in U.S. Pat. No. 5,007,065, semi-monolithic apparatus is used, or as described in U.S. Pat. Nos. 5,027,361, 5,227,911, 4,731,787, 4,797,896, monolithic apparatus is used.
Compared to the discretely structured resonators, these apparatus have various advantages:
With these configurations, doubling efficiencies up to 80% are achieved. However, these configurations can only be tuned within very small wavelength intervals and thus their application is limited to fixed frequency lasers, such as diode-pumped Nd:YAG lasers. Subsequent adjustment or modification of this type of resonator is not possible. In case of partial damage, individual parts can only be replaced at considerable expense, if at all. In addition, the manufacture of the crystals that form the monolithic or semi-monolithic resonator is very complicated (spherically curved surfaces, special coatings, etc.). These crystals are not available on the market and can only be produced in special laboratories. Sometimes these configurations also make use of properties of crystals that are provided only by a few materials, such as a high coefficient for the electro-optical effect in the crystalline material lithium niobate, which is used for tuning the resonator with an electrical voltage. The limitation to such crystals then leads to a severe restriction in the wavelength range that can be doubled by them.
In U.S. Pat. Nos. 5,052,815 and 5,357,537 diode-pumped ring lasers are described whose resonators consist of only two mirrors and one or more refractive elements. They use the flexibility of a discrete structure that allows the adjusting of individual elements or modification by installation of additional optical elements even after the assembly. The resonator losses are reduced by the use of the smaller number of elements, in comparison to classical ring lasers. These advantages are used to create ring lasers with better efficiency and easier manufacture. All of these configurations contain an active laser medium and are used explicitly as laser apparatus.
In another known apparatus (M. Watanabe et al.: "Continuous-wave sum frequency generation near 194 nm . . . ," Opt. Commun. 97 (1993), p. 225), a prism is used for simultaneous, resonant enhancement of two different wavelengths. In this case, the dispersing effect of prisms is used to combine two laser beams of differing wavelength. The two laser beams are amplified in two independent resonators that have a non-linear crystal in their common branch which produces a sum-frequency from the two laser beams.
In the DD 145 588 methods for the shortening or the selection of pulses from CO.sub.2 -- lasers are described, where the incoming laser pulse is divided into a switching and a switched beam by using a beamsplitter. The switching beam is focused into a unit for the generation of the 2. harmonic of the fundamental wave via a bending mirror and a lens. Then the 2. harmonic is guided to a switching crystal, which is antireflection coated for the 2. harmonic and for the fundamental wave. The switched beam is being delayed and directed to a lens with short focal length, thereby focusing it into the focus volume of the switching beam. In this pulse shortening arrangement the length of a beam path is changed by translation of a retro-reflecting prism, without changing the beam path outside the arrangement. By adding some suitable mirrors a resonator can be formed, which can be tuned by translation of the retro-reflecting prism. But the retro-reflecting prism doesn't allow high conversion efficiencies, because additional interfaces have to be introduced into the resonator, causing losses and decreasing the efficiency. The deflection angle of 180.degree. will also decrease the efficiency, because this causes large incidence angles at the resonator mirrors, which have to be curved mirror to form an optical stable resonator. Large incidence angles will cause severe astigmatism, which reduces the efficiency.
In the Journal "Optics Communications", Vol. 53, No. 1, 1985, p. 43-47 a mirrorless configuration for a cw ring dye laser is described, which uses a special prism to form the resonator.
With the described focussing prism and with Littrow prisms an optically stable, astigmatically compensated and tunable resonator can be formed, but due to the large number of interfaces in the resonator frequency conversion with high efficiency or resonance enhancement with usable enhancement factor is not possible with this arrangement.
The purpose of the invention is to develop a method and a device of the described type, so that the frequency conversion of continuous laser radiation, in particular, will occur at high efficiency while avoiding the described disadvantages, and so that an improved tunability will be assured over the greatest possible wavelength range with regard to the incident wavelengths. The improvement in tunability is intended, in particular, to: